Geology Reference
In-Depth Information
These cavities and fillings are believed to be pre-
dominantly of synsedimentary origin and should not
be confused with spar-filled interparticle pores, growth-
cavities in reefs, solution voids and spar-filled fractures,
or mineralized lichens and spar-filled molds of plant
roots found in caliche crusts (Klappa 1978). Birdseyes
occur predominantly in supratidal and intertidal car-
bonates, stromatactis in limestones formed in deeper
and shallow subtidal environments.
Other terms in use for limestones with spar-filled
voids are
dismicrite
(disturbed micrite: Folk 1959), de-
scribing a micritic limestone with less than 1% grains
and containing many birdseyes, and
loferite
(Fischer
1964), characterizing a limestone riddled by spar-filled
shrinkage pores (Sect. 16.1.1.3). Both terms are partly
synonymous with birdseyes limestone but the terms
birdseyes and stromatactis should be reserved for de-
scribing the shape and geometry of the cavities, whereas
loferite may be used for carbonate rocks abundant in
various kinds of fenestral fabrics formed in a peritidal
environment.
vertical tubular voids filled with calcite, in places as-
sociated with birdseyes, are most likely root tubes or
burrows and should be named 'pseudo-birdseyes'
(Shinn 1982). These tubular structures of constant size
often occur in tidal and subtidal fine-grained limestones
(Wu 1983).
5.1.5.2 Fenestral Fabrics
Birdseyes (but also stromatactis-type structures) are
important constituents of fenestral fabrics. Tebbutt et
al. (1965) suggested the name
fenestra
or window to
characterize ... 'a primary or penecontemporaneous gap
in rock framework, larger than grain-supported inter-
stices'. Fenestrae may be open cavities, or may be com-
pletely or partially filled by surface-derived internal
sediment, diagenetic internal sediment (e.g. crystal silt)
or cements. Fenestrae have no apparent support in the
framework of primary grains composing the sediment.
In that respect, they differ from growth-framework
pores or from solution voids.
The term fenestra is now in use for any kind of a
hole of any shape in a rock or unconsolidated sediment
that is not of an intergranular nature (Demicco and
Hardie 1994). It seems, however, better to restrict the
term to fabric-selective textures, and to use the term
void for non-fabric selective cavities.
5.1.5.1 Birdseyes
Birdseyes and birdseye limestones (Pl. 20/7, 8, Pl. 123/
2) are old names used by American field geologists to
describe calcite-filled voids that sparkle in the sunlight.
Apparently, the shape and sparkle of these features re-
minded someone of birds'-eyes. The name was intro-
duced for very small spar-filled voids in Ordovician
micritic shelf limestones (Ham 1952).
Birdseyes occur as isolated bubble-like vugs 1 to
3 mm in diameter, or as planar isolated vugs 1 to 3 mm
high by several millimeter in width forming fenestral
fabrics. The structures are often associated with micro-
bial and algal mats. They are found in carbonate rocks
from the Precambrian to the Recent and originate pre-
dominantly in supratidal, sometimes in upper intertidal
settings.
Terminology of fenestral fabrics
Fenestrae are arranged concordant to stratification,
cross-bedded, or irregularly distributed. Arrangement
Box 5.1.
Proposed origin of birdseyes structures.
Direct or indirect organic interactions
1 Spar-filled interspaces in and between algal and
cyanobacterial mats: Ham 1952; Philcox 1963;
Deelman 1972
2 Cavities supported by algae and enlarged by reced-
ing tidal waters: Wolf 1965
3 Cavities formed by burrowing organisms: Fruth and
Scherreiks 1975
4 Gas bubbles caused by decaying organisms: Korn
1932; Hodgson 1958; Cloud 1960; Shinn 1968
Inorganic
5
Origin of birdseyes
Birdseyes have different origins (Box 5.1). Modes
1, 2, 5 and 6 produce birdseyes arranged more or less
concordant to bedding planes. Irregularly distributed
open-space structures may originate by modes 3, 4 and
7 to 9. Nearly all modes are known from modern shal-
low-marine environments, some have been verified by
experiments (Shinn 1983).
The term birdseyes often is rather loosely used, but
should be restricted to millimeter-scale spar-filled vugs
exhibiting rounded shapes and concentrated in periti-
dal environments (Shinn 1983). Small vertical to near-
Desiccation, shrinkage pores in tidal zones: Fischer
1964
6
Air inclusions formed during subaerial exposure and
subsequent tidal reinduration: Bolliger and Burri
1970; Deelman 1972
7
Different behavior of capillary and non-capillary sedi-
ment in comparison to water: Deelman 1972
8
Leaching of anhydrite
9
Late diagenetic selective recrystallization ('pseudo-
sparitization'): Wolf 1965; Zorn 1971
